The invention relates to a balanced circuit arrangement as defined in the preamble to patent claim 1. A circuit arrangement of this type is generally known.
High-resolution delta-sigma converters are normally used to convert an analog signal, for example the output signal for a capacitor microphone capsule, to a digital signal. For technological reasons, converters of this type operate with a voltage of +5 volt and have symmetric analog inputs. The defined permissible analog input levels for a full control in this case are maximum +/xe2x88x921.25 volt opposite phase for each input connection, which corresponds to a differential signal of +/xe2x88x922.5 volt. An asymmetric analog input signal, for example as provided by a capacitor microphone capsule behind the impendance converter, therefore must always be inverted as well. For this, FIG. 1 shows a balanced circuit arrangement according to prior art.
In FIG. 1, the output signal S1 from a signal source 1, for example a microphone capsule, is connected to the non-inverting input of a first amplifier 2. So that the first amplifier 2 has an amplification of V=+1, its output signal S2 is fed back in the form of a negative feedback to the inverting input of amplifier 2. The signal S2 is also supplied to the non-inverting input of an analog/digital converter 4. In addition, the output signal S2 from the first amplifier 2 is connected via a series resistor (R2) to the inverting input of a second amplifier 3. The non-inverting input of the second amplifier 3 is connected to ground or to a reference potential. The output signal S3 from the second amplifier 3 is fed to the inverting input of the analog-digital converter 4 and is also fed back to the inverting input of the second amplifier 3 via a resistor R1, in the form of a negative feedback. The negative feedback resistor R1 has the same value as the series resistor R2. As a result of these connections, the second amplifier 3 has an amplification degree of V=xe2x88x921 for its input signal S2. The output rate of output signal S3 from the second amplifier 3 is equal to that of the output signal S2 from the first amplifier 2 and is inverted as compared to the signal S2. The digital signal S4 generated at the output of the analog/digital converter 4 represents the differential signal between the input signals S2 and S3 of the converter 4 and thus corresponds to double the value of signal S2.
The balanced circuit arrangement according to FIG. 1 has the following disadvantages:
As referred to the symmetric input of the analog/digital converter 4, the signal S1 from the signal source 1 is amplified by the factor 2, so that only a maximum source signal level of +/xe2x88x921.25 volt can be processed. However, the peak amplitudes processed in modem analog capacitor microphone capsules are considerably higher.
The second amplifier 3 adds a disproportionate share of its own noise to the noise from the first amplifier 2. The reason for this is that the noise developing in the second amplifier 3 always appears as if it were fed into its inverting input. Thus, while the amplification degree of the second amplifier 3 for the useful signal is adjusted with the aid of resistors R1, R2 to V=xe2x88x921, the noise from the second amplifier 3 is amplified in accordance with the formula (R1+R2)/R2=2, in the same way as for a signal fed into its non-inverting input.
Accordingly, it is the object of the invention to create a balanced circuit arrangement that can process higher maximum levels of the source signal and generates less noise.
This object is solved according to the invention with the characterizing features in claim 1.
Advantageous embodiments and modifications of the balanced circuit arrangement according to claim 1 follow from the dependent claims.